1. Field of Invention
This invention relates to electronic bridge networks as used for the measuring of complex electrical impedance values comprising resistive (real) and reactive (imaginary) components.
2. Description of Prior Art
FIG. 1 illustrates a bridge configuration that is most commonly used in low cost radio frequency (rf) bridge based measuring instruments. Instruments of this type determine the complex electrical impedance magnitude (|ZX|) and the associated reflection coefficient magnitude (|xcfx81|) for the complex impedance value being measured. Magnitude values for the real (RX) and unsigned reactive (XX) components as well as Voltage Standing Wave Ratio (VSWR), as taught by Paul""s seminal U.S. Pat. No. 2,323,076 (1940), are then derived.
Referring to FIG. 1, Vsource is an alternating current (a.c.) voltage source at a frequency of interest for the measurement. ZX is a complex impedance whose value is unknown and having, in general, both resistive (real) and reactive (imaginary) components. ZX may be any two terminals of a general N-port network, not to exclude an antenna with or without feed-line. RO may, in general, be any value but is normally selected to be equal to a standard characteristic impedance, typically 50 or 75 ohms resistive. Voltages |VD|, and |V1| are indicated by measuring devices M1 and M2 respectively, simple a.c. voltmeters with magnitude determination only. Also, measuring devices M3 and M4, simple a.c. voltmeters with magnitude determination only, indicate voltages |VBxe2x88x92V2| and |V2| respectively. It follows that a computing device (not illustrated) could utilize the four measured values of FIG. 1 to calculate the following useful quantities:
Complex Impedance Magnitude=|ZX|=RO|V2|/(|VBxe2x88x92V2|),
Voltage Across the Horizontal Bridge Diagonal=|VD|=|V2xe2x88x92V1|,
Reflection Coefficient Magnitude=|xcfx81|=|(ZXxe2x88x92RO)/(ZX+RO)|=|VD|/|V1|,
Voltage Standing Wave Ratio (VSWR)=(1+|xcfx81|)/(1xe2x88x92|xcfx81|),
Resistive Component of ZX=RX={(|ZX|2+RO2)(1xe2x88x92|xcfx81|2)}/ {f2RO(1+|xcfx81|2)} and
Unsigned Reactive Component of ZX=XX=(|ZX|2xe2x88x92RX2)xc2xd
where |ZX|, RO, |xcfx81|, VSWR, RX, and XX are real unsigned numbers.
BACKGROUND-LIMITATIONS OF THE PRIOR ART
Measuring accuracy and bandwidth for the circuit of FIG. 1 are primarily limited by measuring devices M1 through M4, the accuracy and purity of the resistive bridge elements and stray inductances, capacitances and resistances associated with the physical embodiment of FIG. 1. The measuring devices depicted in FIG. 1 as M1 through M4 typically comprise rf detection (rectification) diodes, in series with an appropriately valued capacitor, connected between the measurement nodes as illustrated in FIG. 2 and contribute to measuring error in several ways.
In FIG. 2, voltages |VD|, |V1|, |VBxe2x88x92V2| and |V2| are developed across four capacitors, C1 through C4, respectively, after being detected by diodes D1 through D4, respectively. Not shown in FIG. 2, the detected voltages may be indicated on simple direct current (d.c.) voltmeters or converted to a digital format with an Analog to Digital Converter (ADC) for processing. The best embodiment of rf diodes D1 through D4 is based on zero bias schottky or back (tunnel) diodes, special diodes perfected for use as small signal radio frequency detectors. Even the best embodiment for the rf diodes will exhibit detection inaccuracies due to non-linear diode forward conduction characteristics, especially at small voltage levels normally experienced across the bridge horizontal diagonal (|VD|=|V2xe2x88x92V1|). When detecting small voltage levels, the non-linear diode characteristics cause dead zones where no useful voltage is detected thus causing measurement drop-outs. Variations of the diode junction voltages, induced by ambient temperature fluctuations, will further add to inaccuracies in detected voltages. Forward d.c. biasing of the diodes as well as compensation for the above cited errors, in the computing device for determining |ZX|, |xcfx81|, VSWR, RX or unsigned XX, provides some improvement but accuracy is still severely limited for low VSWR measurements and for measuring of |ZX| values near RO.
Referring again to FIG. 2, a second measuring limitation of the prior art is the loading effect and resulting measurement distortion and accuracy degradation caused by the measuring device being connected in parallel with the various bridge sides. This alters the effective value of the bridge resistances, RO, and even the value of the impedance being measured, ZX, in a nonlinear manner difficult to characterize for compensation purposes.
A further limitation of the prior art is the inability to determine a sign for the derived reactive component of the complex impedance being measured. Attempts have been made in prior art to overcome this shortfall by detecting a relative phase difference between (VBxe2x88x92V2) and V2 as shown in FIG. 2. Unfortunately, connecting a phase detector across the impedance being measured produces further unwanted loading and attendant measurement distortion and accuracy degradation.
Prior art measuring accuracy of |ZX| values, due to the above limitations, is typically xc2x110% to xc2x135% for VSWRs of 2:1 or less with nominal measuring ranges of 5 to 1000 ohms. Multiple instruments are required to cover the 1 to 500 MHz frequency range and coverage above 500 MHz is not generally available. A typical ambient operating temperature range is 10 to 35 degrees Centigrade.
BACKGROUND-OBJECT AND ADVANTAGES
Accordingly, several objects and advantages of the new invention are:
(a) to provide a low-cost complex rf impedance measuring capability with improved accuracy compared to prior art,
(b) to provide a low-cost complex rf impedance measuring capability with increased bandwidth compared to prior art,
(c) to provide a low-cost complex rf impedance measuring capability with increased measuring range compared to prior art,
(d) to provide an rf bridge in which there is negligible loading of the unknown impedance being measured by the measuring device,
(e) to provide an rf bridge capability that is linear in response from low to high values of measured quantities without the dead zones associated with attempted detection of voltages below diode thresholds commonly found in prior art,
(f) to increase rf bridge measuring accuracy, compared to prior art, over wider ambient temperature variations,
(g) to provide for nullification of measuring device capacitive loading effects on the rf bridge and
(h) to provide an rf bridge capability with constant measuring accuracy over a wide measuring range.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.
My invention, a radio frequency bridge, useful for measuring complex values of an unknown impedance or associated complex reflection coefficient and VSWR value, comprises an rf source, a novel asymmetrical bridge, a measuring device (comprising two logarithmic amplifiers, a difference amplifier and a phase detector), a computing device and a display or interface device. The complex impedance may be any two terminals of a general N-port network not to exclude an antenna with or without feed-line. The invention is more accurate than prior art when measuring complex impedance values having magnitudes from approximately five ohms to an upper value limited only by the dynamic range of the logarithmic amplifiers, normally greater than 4000 ohms.
Referring again to FIG. 3, the invention employs a novel asymmetrical bridge with a measuring device, improved over prior art, to increase accuracy, bandwidth and measuring range. It has the great advantage of maintaining constant measuring accuracy over a wide measuring range. The bridge is a wide band design for complex impedance measuring, implemented in a manner to minimize bridge associated stray inductances, capacitances and resistances. The bridge may be frequency compensated, over a wide range of frequencies, to nullify the effects of the measuring device""s logarithmic amplifier input capacitance.
A computing device can provide a complex value (magnitude and phase angle) as well as real values for the resistive and signed reactive component for the complex impedance being measured. A complex value for the associated reflection coefficient and a real VSWR value can also be calculated as well as other desired quantities. All computed quantities are either displayed on a display device or interfaced for subsequent processing or display.